The present invention relates to a surface acoustic wave device and a process for producing the same. More particularly, the invention relates to a piezoelectric substrate having uniquely shaped ends and a cutting method for providing such unique shaped ends.
The surface acoustic wave device has interdigital (comb-shaped) electrodes on a piezoelectric substrate for converting electrical signals to surface acoustic waves, and vice versa.
FIG. 1 shows the conventional surface acoustic wave device having a piezoelectric substrate 1 which is typically formed of LiNbO.sub.3 (lithium niobate), LiTaO.sub.3 (lithium tantalate) or other piezoelectric ceramics. The substrate 1 has on its surface an input electrode 2 which converts input electrical signals to surface acoustic waves 4, and an output electrode 3 which picks up the propagating surface acoustic waves 4 after conversion to electrical signals. While most of the input electrical signals that have been converted to the surface acoustic waves 4 at the input electrode 2 propogate along the surface of the substrate toward the output electrode 3 on the right side of FIG. 1, part of the waves propagate toward the left end of the substrate 1, where that part is reflected back to the output electrode 3. A portion of the surface acoustic waves 4 that have propagated to the output electrode 3 directly from the input electrode 2 pass through the output electrode 3 and reach the right end of the substrate 1 where that portion is reflected back to the output electrode 3. These surface acoustic waves reflecting at either end of the substrate cause ripples in the amplitude characteristics or group delay characteristics in the passband of an elastic wave filter.
This problem is conventionally solved by the following two methods. First, the two ends of the substrate are cut obliquely as shown in FIG. 1 so that the elastic waves reflecting from each end will not return directly to the output electrode 3. The effectiveness of the oblique ends is enhanced by providing them with an absorbing material 5 that absorbs the unwanted surface acoustic waves as much as possible.
By cutting both ends of the substrate obliquely, surface acoustic waves reaching either end can be reflected in such a direction that they will not directly reach the output electrode 3. A portion of the surface acoustic waves 4 created by the input electrode 2 may be reflected at either longer side of the substrate (FIG. 1) before reaching the output electrode 3, but this portion of the elastic waves has a longer travel time through the absorbing material 5 than the usual elastic waves reflecting at both ends of the substrate, and the resulting great propagation loss leads to a low signal level of the reflected waves.
The above method is effective for the purpose of suppressing the surface acoustic waves that have reflected at both ends of the substrate. However, this method requires a parallelpipedic chip, and the materials cost of each chip is high because the number of parallelpipedic chips that can be sliced from a single piezoelectric substrate wafer is smaller than that of the usual square or rectangular chips. Alternatively, square or rectangular chips may first be sliced from the wafer and both ends of each chip are then cut obliquely to a parallelpipedic shape. However, this technique has one extra step as compared with simply slicing a parallelpipedic chip from the wafer.